Acoustical resistance for pressure type microphones



Dec. 4, 1956 l I I I I I I I I H. F. OLSON ET AL ACOUSTICAL RESISTANCE FOR PRESSURE TYPE MICROPHONES Filed March 31, 1953 INVENTORS A RY F: DL5UN| .JDHN PRESTON /I TTORNE I ACOUSTICAL RESISTANCE FOR PRESSURE TYPE MICROPHONES Harry F. Olson, Princeton, and John Preston, Metediconk, N. 1., assignors to Radio Corporation of America, a "corporation of Delaware Application March 31, 1953, Serial No. 345,979

5 Claims. (Cl. 179-1155) United States Patent O to form the backing member for the slit opening. An

obvious difliculty with this type of system is that very close tolerances must be imposed upon the aforementioned spacing between the top plate and the washer which spacing is difficult and expensive to maintain in production. The spacing is of the order of one-thousandth of an inch. A variation of this spacing of ten percent will vary the acoustical resistance by thirty-three percent. That is, a variation of one ten-thousandth of an inch will vary the acoustical resistance by thirty-three percent. Thus it will be seen that this type of acoustical resistance is extremely expensive to manufacture because it requires absolutely plane surfaces and exceedingly precise spacing. Microphones employing this type of acoustical resistance have been manufactured and sold. However, the cost of the microphones employing this type of acoustical resistance is so high that as a result a relatively small number of such microphones have been sold.

It is, therefore, an object of the present invention to provide a novel acoustical resistance for pressure type microphones.

It is also an object of the invention to provide a very simple and inexpensive and easily fabricated and assembled acoustical resistance.

It is a still further object of the invention to provide a. simple acoustical resistance for a pressure microphone which will exhibit a definite value of acoustical resistance without adjustments after the assembly of the microphone in order to obtain the proper characteristics.

In accordance with the invention, there is provided an acoustical resistance for a pressure type microphone comprising a multiple pipe ortube which may take the form of a knurled washer or a properly dimensioned fine wire metallic screen. The washer or screen resistance is introduced into the microphone in the area which normally includes the slit between the top plate and the washer member adjacent the diaphragm of the microphone.

The novel features which are considered characteristic of the invention are set forth with particularity in the appended claims, but for a better understanding of the invention itself both as to its organization and method of operation together with other and further objects and advantages, reference may be had to the following description of certain specific embodiments shown merely for illustration taken in connection with the accompanying drawing, in which:

Fig. 1 is a sectional view of a dynamic pressure microphone employing a slit as the acoustical resistance element;

Fig. 2 is a view similar to Fig. 1, illustrating the use of silk cloth as the acoustical resistance element;

Patented Dec. 4, 1956 Fig. 3 is a view similar to Fig. 1, illustrating the use of the novel knurled washer in accordance with the pres-' ent invention as the acoustical resistance;

Fig. 4 is a view similar to Fig. 1, illustrating the use of a wire mesh screen also in accordance with the present invention as the acoustical resistance;

Fig. 5 is a plan view of the washer element shown in Fig. 3; and

Fig. 6 is a plan view of the wire mesh screen element shown in Fig. 4.

An acoustical resistance can be obtained from any passage of such dimensions that viscosity plays an important role. For example, the acoustical impedance of a narrow slit is given by the equation 1 a: CM +1 5111 where ,IL VISCOSltY coefiicient, 1.86 ,1O- for air,

Referring to Equation 1, it will be seen that the ratio of acoustical resistance to acoustical reactance can be increased by making d smaller. Thus it is possible to make a slit practically a pure acoustical resistance with very little acoustical reactance. The magnitude of the acoustical resistance can be adjusted by means of l and w. A dynamic pressure microphone 10 employing a slit 12 as the acoustical resistance element is shown in Fig. l. The slit 12 is located behind the diaphragm 15, between a washer element or backing member 14 and the top plate 16 of the microphone.

In order to overcome the above described disadvantages of the slit type of acoustical resistance, there has been developed a silk cloth 17 as a substitute for the slit type acoustical resistance as shown in Fig. 2., The silk cloth acoustical resistance is fully disclosed in U. S. Patent 2,106,224 granted to H. F. Olson on January 25, 1938. The use of the silk cloth is a very simple and inexpensive expedient. A large majority of diaphragm pressure type microphones employ silk or other cloth as the acoustical resistance material. The mechanism of acoustical resistance in cloth is very complex. There are narrow passages in the cloth which introduce resistance due to viscosity. There is also a flow past the fibers of the material which also introduces viscosity. One of the difficulties in the cloth type of acoustical resistance is that the magnitude of the acoustical resistance is diificult to control. The accepted method is to assemble the microphone and obtain the response frequency characteristic curve. If this response frequency characteristic is satisfactory the microphone is passed. If it is not satisfactory the microphone is torn down and the cloth reassembled. In this connection the acoustical resistance is also measured by pressure against volume velocity means. For example sometimes ninety percent of the microphones are satisfactory. Sometimes only fifty percent are passed. Nevertheless, this latter type of resistance element is cheaper to manufacture than the slit type.

Continued experimentation has resulted in discovery of the simple and novel solution to the aforementioned problem disclosed herein. In accordance with this discovery, use is made of a multiple fine pipe or tube type of acoustical resistance. The acoustical resistance of a fine tube is given by the equation p Z 8 1 v ZA TR2 where R=radius of the tube or pipe, in centimeters,

,u=viscosity coeflicient, 1.86 for air, w==21rf, f=frequency, in cycles per second, l=length of the tube, in centimeters, =density of air, in grams per cubic centimeter.

By the selection of R in Equation 2 any ratio of acoustical resistance to acoustical reactance can be obtained. Referring to this latter equation, it will be seen that by making R suificiently small it is possible to obtain prac tically a pure acoustical resistance with a very small acoustical reactance.

Referring now to Fig. 5, there is shown a novel means for obtaining a fine tube of exact dimensions which may be made to serve as the acoustical resistance. The acoustical resistance comprises a washer 18 provided with criss-cross grooves or knurling 24 thereon. The washer 18 is secured against the top plate 16 of the interior of the microphone 10 and surrounds the central magnet structure adjacent the inner pole piece 19 and the outer pole piece 20 as illustrated in Fig. 3. The central magnet structure is secured to and supported by the support member 21. A conductor comprising a voice coil is secured to a sleeve member which is adapted to,surround the central magnet structure for cooperation therewith. When the diaphragm of the microphone 10 moves, air is pushed through a multiplicity of narrow tubes 22 formed by the tortuous paths resulting from the knurling 24 cut into the washer 18. The knurled washer 18, may for example, be struck from a die. Thus it will be seen that every knurled washer will be like every other one struck by the die. When the knurled washer 18 is placed against the top plate 16 of the microphone 10 the multiple tubes 22 will be substantially the same for all assemblies. Consequently, the acoustical resistance will be substantially the same. Furthermore, this latter assembly is much cheaper and simpler to produce than the silk cloth previously mentioned. The magnitude of the acoustical resistance depends upon the radial length of the washer and the number of ridges per inch and the angle of the knurl.

It has been found that a washer 26 which has been cut from a metal screen or mesh of predetermined weight also forms an acoustical resistance as illustrated in Fig. 6. In this case as with the knurled washer, a multiplicity of fine reticulations or tube-like paths 28 from the inside out results when the screen is placed between the top plate 16 and the washer 14 as shown in Fig. 4. The value of acoustical resistance can be adjusted by means of the size of the wire, the number of wires per inch and the path length from the inside to the outside. The screen washer being simpler and easier to manufacture than the knurled washer, provides another and more readily available acoustical resistance which may also be readily introduced into the microphone as was the knurled washer above described. The screen type acoustical resistance allows the acoustical resistance to be duplicated con sistently and accurately regardless of the number of screens required.

What is claimed is:

1. A microphone comprising a diaphragm, a pair of pole pieces having an air gap therebetween, a conductor secured to said diaphragm and disposed within said air gap, acoustical resistance means being disposed adjacent one end of said air gap and between said pole pieces, said acoustical resistance means comprising a reticulated annular member having one surface thereof adjacent at least one of said pole pieces, said member providing crisscross air chambers and passages between said one surface thereof and the surface of said pole piece adjacent thereto, and a backing member secured to the surface of said annular member opposite to said one surface.

2. A microphone comprising a diaphragm, a pair of pole pieces having an air gap therebetween, a conductor secured to said diaphragm and disposed within said air gap, a backing member, and an acoustical resistance secured adjacent said pole pieces by said backing member, said acoustical resistance comprising a knurled washer having criss-cross air passages.

3. A microphone comprising a diaphragm, a pair of pole pieces having an air gap therebetween, a conductor secured to said diaphragm and disposed within said air gap, a backing member, an acoustical resistance secured adjacent said pole pieces by said backing member, said acoustical resistance comprising an annular screen member having criss-cross air passages.

4. A microphone comprising a diaphragm, a pair of pole pieces having an air gap therebetween, a conductor secured to said diaphragm and disposed Within said air gap, a backing member, and an acoustical resistance secured adjacent said pair of pole pieces by said backing member, said acoustical resistance comprising a washer inner periphery and an outer periphery, said inner periphery being secured adjacent one of said pole pieces and said outer periphery secured adjacent the other of said pole pieces, said washer having a multiplicity of criss-cross passages extending from said outer periphery towards said inner periphery.

5. In a dynamic microphone, the combination comprising a magnetic structure having inner and outer pole pieces, said pole pieces providing an air gap therebetween, a diaphragm, a sleeve member attached to said diaphragm, a voice coil secured to said sleeve member, said sleeve member and said voice coil being disposed around said inner pole piece within said air gap, a backing member disposed around said inner pole piece below said air gap and providing with said outer pole piece a passageway extending from said air gap, means providing an acoustical resistance for said microphone, said means including a multiplicity of criss-cross tube like passages, and said means being disposed in said passageway between said backing member and said outer pole piece with said tube like passages thereof being directed along said passageway.

References Cited in the file of this patent UNITED STATES PATENTS 1,915,358 Giles June 27, 1933 1,952,167 Hasbrouck et al Mar. 27, 1934 2,024,271 Blumlein et al Dec. 17, 1935 2,549,963 De Boer et al Apr. 24, 1951 

